Integrins are a family of cell adhesion/signaling molecules that play an important role in development, wound repair, angiogenesis, immunity, and tissue integrity. Regulation of specific integrins is a potentially powerful therapeutic target for diverse diseases including cancer and autoimmunity. A remarkable characteristic of integrins is the degree of cellular regulation that can be achieved without altering protein expression levels. For example, the integrin LFA-1 is initially inactive on resting lymphocytes, but adhesion activity is rapidly increased by exposure to chemokines or antigen. Changes in LFA-1 affinity for ligand are not the primary mechanism for regulation. In contrast, there is mounting evidence that interactions of integrins with the cytoskeleton is important. Our hypothesis is that integrin activity is regulated by a multistep cascade with the following major steps: 1) the integrin is initially attached to the cytoskeleton to prevent diffusion limited reaction with ligands, 2) activation releases the integrin from cytoskeletal constraints to increase ligand binding, 3) ligand binding induces a conformational change in the integrin, and 4) the ligated integrin binds cytoplasmic factors that regulate local mechanical properties to enhance two dimensional affinity of integrin- ligand interaction. We will test these hypotheses by examining the physiological chemistry of integrin interactions (bonds) using a novel fluorescence based approach. In Aim 1 we will determine how different biologically important modes of lymphocyte activation affect LFA-1 engagement in cell substrate contact areas. In Aim 2 we will determine how different modes of lymphocyte activation affect LFA-1 lateral mobility as an assay for release from the cytoskeleton. In Aim 3 we will determine the most effective strategy to probe integrin cytoskeletal interactions through expression of exogenous cytoplasmic domain.